The present invention relates broadly to a magnetic reference apparatus, and in particular to a thin-film magnetometer compass.
Thin magnetic films have been produced by depositing a nickel-iron alloy on a smooth substrate to a thickness of a few hundred to several thousand Angstroms. A number of deposition processes, including evaporation in a vacuum and electroplating may be employed. In the evaporative process the deposition of the magnetic material on a glass substrate may be made directly, whereas electroplating on a glass substrate requires the application of a conductive coating on the glass prior to deposition. In general, the characteristics discussed hereinafter apply to films deposited by either of these processes, although in electroplated films consideration must be given to the possible high-frequency eddy-current effects in the required conductive underlayer.
In general, predictable and stable magnetic properties of the films are obtained by choosing an alloy composition which yields minimum magnetostriction coefficient. For the nickel-iron film, the optimum composition appears to be approximately 83% Ni, 17% Fe. It has been found experimentally that if the actual composition of the film differs from this ratio by more than a few percent, the film magnetic properties are unduly sensitive to stresses induced by thermal expansion of the substrate or by external forces.
Films of thicknesses up to at least 3,000 Angstroms exhibit the capability of existing as a single domain, the magnetization of which can be rotated from a preferred or easy direction of magnetization by the application of external fields. This easy axis anisotropy is produced in the films by the presence of a large uniform field during the evaporation process which causes the magnetic domains of the alloy to align in a preferred direction. The magnetic characteristic of thin films in the preferred direction exhibits a substantially rectangular hysteresis loop. In a direction transverse to the easy direction, often referred to as the hard direction or axis, the magnetic characteristic is substantially linear loop.